High-chromium (at least 12% by weight) white cast iron is well known for its high resistance to abrasion. There are several different types of abrasion. Each has a unique way of attacking the surface exposed to it.
One is erosive abrasion, in which minute, hard, mainly mineral particles strike the casting surface at varying velocities and angles ranging from 0.degree.-90.degree.. Slurry pump parts, parts of earth moving equipment, slurry classifier shoes, log washer teeth and chute liners are just some of the castings subjected to erosive abrasion.
Another type of abrasion is one where the abrasive strikes the casting surface with enough force to tear out metal particles. This is a process accompanied by high dynamic loads. Crushing machine jaws, hammer mills, working parts of dredges and plungers are among the castings subjected to this type of abrasion.
Another wear-inducing type of abrasion can be found where the abrasive is being crushed, or pulverized between two moving, working surfaces. Here the abrasive penetrates the casting surface with a significant perpendicular force. This type of wear is found in pulverizer ball mills, plate crushers, roll mills, and the like.
In the above-mentioned wear conditions, the parts of the machines subject to wear are most often made of high-chromium white cast iron having eutectic metal carbides (M.sub.x C.sub.y) precipitated in a very hard, durable, and at the same time, tough matrix.
Hardness is the one mechanical property of high-chromium white cast iron which the individual users as well as the manufacturers look for and examine most closely, although other (non-standard) tests to determine strength, impact resistance and fracture toughness are sometimes employed as well.
It is well known that castings of high-chromium white cast iron possess good resistance to the many types of abrasion to which such castings are subjected. This abrasion resistance is achieved during solidification of the casting by precipitation of eutectic carbides of great hardness in a matrix which should also be hard and durable. The carbides and the matrix act essentially independently of each other.
At the present time, high-chromium white cast irons are further hardened through some type of high temperature heat treatment process. By far the most widely used and proven method involves heating the castings up to 900.degree. C.-1060.degree. C. and holding them there for 1-4 hours, depending on chemical considerations of different compositions. This temperature is recommended for precipitation of secondary carbide particles in the austenite in very high density. The precipitation lowers the amount of carbon dissolved in the austenite and raises the M.sub.s temperature. This permits transformation of substantial amounts of austenite to martensite during cooling to room temperature. Usually air quenching is used. Following the air quenching, it is advisable to stress-relieve the castings at 200.degree. C.-260.degree. C. In this process, the matrix contains strongly precipitated secondary carbides in very high density, which is the reason for its brittleness. The large temperature gradients involved in these processes also has a negative effect on the toughness of the castings.
Another method of hardening high-chromium white cast iron is a subcritical heat treatment, consisting of maintaining the casting at 450.degree. C.-600.degree. C. for 6-12 hours. This type of hardening has as its goal the reduction of retained austenite by transforming it into the harder ferrite-carbide phase. The amount of austenite that will transform depends largely on the composition and the prior cooling rate of the casting while in the mold. This low temperature transformation of austenite is, therefore, not feasible except for very slowly cooled castings. Usually, high chromium-molybdenum alloy irons, which are cast in thicknesses which cool quickly in the mold, do not allow sufficient precipitation of secondary carbides and as a result have as much as, or more than, 50% austenite in the matrix structure. The transformation of this austenite by subcritical heat treatment is very difficult because of the super-saturation of the matrix with carbon and alloying elements. This type of hardening is also associated with the precipitation of ferritic secondary carbides which reduce the durability of the matrix. In addition, this type of hardening causes excessive tempering of the preliminary martensite, lowering its hardness and its abrasion resistance. No one has exceeded 650-680HB with this method.
High-chrome molybdenum white cast irons, which undergo high temperature (900.degree. C.-1060.degree. C.) or subcritical (450.degree. C.-600.degree. C.) heat treatments, gain in abrasion resistance due to the hardening of the matrix. Castings which have complicated shapes and varying section thicknesses are used as-cast, without heat treating, because of the danger of cracking due to the thermal stresses developed during the treatment. The as-cast hardness of the castings 450-500 HB) is low, and the abrasion resistance is also lowered. Castings with large section thicknesses and large masses, such as those used in crushers, pulverizers and steel mill rollers, when made of high-chromium-molybdenum white iron having some nickel in the composition and subjected to a high temperature heat treatment, have a martensitic-austenitic-bainitic matrix. The bainite is present in the matrix because castings of such large section thicknesses cannot be cooled fast enough, due to the large temperature difference between the casting surface and center and also due to the accompanying high thermal stress. However, an increased addition of nickel increases the amount of retained austenite, which simultaneously lowers the hardness and increases the likelihood of failure due to abrasion and spalling. High-chromium molybdenum white cast iron having additional nickel or copper, and properly cooled in the mold, almost always has a martensitic-bainitic structure, with some perlite and retained austenite. Chrome-molybdenum-nickel irons designed for high temperature heat-treatment, usually contain a limited amount of nickel and copper, usually totalling together a maximum 1.5%, due to the strong stabilization of austenite and its retention in the matrix in the form of the so-called "retained austenite".
High-chromium white irons hardened through high temperature heat treatment contain molybdenum as the chief perlite-suppressing (hardenability-increasing) alloying element. Molybdenum, a potent carbide-former as well as a ferrite-former, is tied up in carbides to a major extent. Molybdenum has a moderate influence in retarding the transformation of austenite to perlite in the range of the A.sub.c1 temperature (subcritical) and the M.sub.s temperature (beginning of martensitic transformation). However, it has no influence on the bainitic transformation.
High-chromium-molybdenum white irons, when heat treated at temperatures of 900.degree. C.-1060.degree. C. and then cooled by air quenching at a rate greater than the critical cooling rate, become hardened. The matrix attains a martensitic structure with retained austenite, which is super-saturated with carbon. Castings made of high-chromium-molybdenum and high chromium-molybdenum-nickel white iron, cooled in sand molds, contain martensitic-bainitic structure, or after fast cooling, martensitic structure with large amounts of retained austenite, which are super saturated with carbon. The transformation of super-saturated retained austenite to martensite by sub-critical heat treatment is very difficult.